1. Technical Field
The present invention relates to an active matrix display device in which driving of luminescent elements such as LEDs (light emitting diodes) or EL (electroluminescent) elements which emit light when a driving current is passed through an organic semiconductor film is controlled by thin film transistors (hereinafter referred to as TFTs). More specifically, the present invention relates to a technique of optimizing the layout to improve the display performance.
2. Description of Related Art
Active matrix display devices using current-controlled luminescent elements such as EL elements or LEDs have been proposed. Any types of luminescent elements used in display devices of this type have the capability of emitting light. Therefore, backlight is not required in display devices of this type unlike the liquid crystal display devices.
FIG. 13 is a block diagram illustrating an example of such an active matrix display device in which carrier injection type organic thin film EL elements are employed. The display device 1A shown in this figure includes various elements formed on a transparent substrate, such as a plurality of scanning lines "gate", a plurality of data lines "sig" extending in a direction crossing the direction in which the plurality of scanning lines "gate" extend, a plurality of common power supply lines "com" extending in a direction parallel to the data lines "sig", and pixel regions located at respective intersections of the data lines "sig" and the scanning lines "gate". To drive the data lines "sig", there is provided a data line driving circuit 3 including a shift register, level shifters, video lines, and analog switches. Similarly, to drive the scanning lines, there is provided a scanning line driving circuit 4 including a shift register and level shifters. Each pixel region 7 includes a first TFT 20 having a gate electrode to which a scanning signal is supplied via a scanning line, a holding capacitor "cap" for holding an image signal supplied from a data line "sig" through the first TFT 20, a second TFT 30 having a gate electrode to which the image signal held by the holding capacitor "cap" is supplied, and a luminescent element 40 into which a driving current flows when the luminescent element 40 is electrically connected to a common power supply line "com" via the second TFT 30.
In each pixel region, as shown in FIGS. 14(A) and 14(B), the first TFT 20 and the second TFT 30 are formed using two respective island-shaped semiconductor films wherein one of the source/drain regions of the second TFT 30 is electrically connected to an interconnecting electrode 35 via a contact hole formed in a first interlayer insulating film 51 and the interconnecting electrode 35 is electrically connected to a pixel electrode 41. At upper layers above the pixel electrodes 41, there are provided a hole injection layer 42, an organic semiconductor film 43, and an opposite electrode "op", which are formed in a multilayer structure. The opposite electrode "op" extends across the data lines "sig" and other lines over a plurality of pixel regions 7.
The other one of the source/drain regions of the second TFT 30 is electrically connected to the common power supply line "com" via a contact hole. On the other hand, in the first TFT 20, one of the source/drain regions is electrically connected to a potential sustaining electrode "st" which in turn is electrically connected to an extension 310 of the gate electrode 31. A semiconductor film 400, which is doped with an impurity so that it exhibits conductivity, is disposed below the extension 310 such that the semiconductor film 400 and the extension 310 face each other via a gate insulating film 50. As a result, a holding capacitor "cap" is formed with the extension 310, the gate insulating film 50, and the semiconductor film 400. The semiconductor film 400 is electrically connected to the common power supply line "com" via a contact hole formed in the first interlayer insulating film 51. The holding capacitor "cap" holds the image signal supplied from the data line "sig" via the first TFT 20 so that the gate electrode 31 of the second TFT 30 is maintained at a potential corresponding to the image signal even after the first TFT 20 is turned off. As a result, the driving current keeps flowing from the common power supply line "com" into the luminescent element 40 and thus the luminescent element 40 keeps emitting light.
However, in the display device described above, unlike liquid crystal display devices, the opposite electrode "op" opposing the pixel electrodes 41 is formed on the same transparent substrate 10 on which the pixel electrodes 41 are formed, such that the opposite electrode "op" extends over the entire surface of the transparent substrate 10 or over the plurality of pixel regions 7, and thus there is only a second insulating film 52 between the opposite electrode "op" and the data lines "sig". As a result, the data lines "sig" have a large parasitic capacitance which causes the data lines "sig" to have a large load. Similarly, a large parasitic capacitance is present between the opposite electrode and interconnection layers included in the data line driving circuit 3 or the scanning line driving circuit 4, because the opposite electrode "op" extends over the data line driving circuit 3 and the scanning line driving circuit 4. As a result, the data line driving circuit 3 also has a problem of a large load caused by the large parasitic capacitance.